1. Field of the Invention
This invention relates to a ferrule for an optical connector to be used in connecting single-mode fibers together. More specifically, the invention relates to an economical plastic ferrule for an optical connector, the ferrule being very small in the amount of eccentricity and in the dimensional change of the outer diameter of its cylinder, and the ferrule being capable of achieving reduction in the assembly cost and parts cost; and a method for the production of the ferrule.
2. Description of the Related Art
With the progress of optical communication technologies, it is becoming possible to introduce optical fibers into the household level and provide varieties of communication services. To realize such an optical communication network for use among general subscribers, there is need for a low cost, economical optical connector.
In connecting together optical fibers for use in optical communication, two ferrules 01A and 01B having optical fiber cords 03A and 03B inserted therein are arranged and aligned in a split alignment sleeve, as shown in FIG. 1. FIG. 2 is a sectional view of a ferrule used for this purpose, and shows a schematic structure of the ferrule. The ferrule has a coated optical fiber guide hole 04, and a fiber insertion hole 05 at the front end. The ferrule also includes a cylindrical portion 06 directly related to connection, a holding portion 07 for allowing the ferrule to be stably held in an optical connector, and an inserting portion 08 for allowing an optical fiber to be inserted into the ferrule.
The ferrule serves as a guide for holding an optical fiber and inserting it into an optical connector when the optical fiber is connected to another optical fiber. Very high dimensional accuracy as shown in Table 1 is required in the diameter of the optical fiber insertion hole, the amount of eccentricity of the optical fiber insertion hole, the outer diameter of the ferrule, and the roundness of the outer diameter.
TABLE 1 ______________________________________ Dimensional requirements for ferrule Item Dimensional tolerance ______________________________________ Outer diameter 2.499 .+-. 0.0005 mm Amount of eccentricity of &lt;0.7 .mu.m optical fiber insertion hole Size of optical fiber 125 + 1 .mu.m insertion hole - 0 .mu.m Roundness of outer diameter &lt;0.5 .mu.m Cylindricity of outer diameter &lt;0.5 .mu.m Surface roundness &lt;0.5 .mu.m ______________________________________
So far, a ferrule has been prepared by precision cutting and grinding a ceramic or metal material piece by piece. This has posed the problems of a low manufacturing efficiency and high parts costs.
To solve these problems, there have been attempts to produce a ferrule by plastic molding. For example, Yoshizawa et al. prepared a plastic ferrule by transfer molding of a phenolic resin (Research & Development Report, Vol. 32, No. 3, pp. 831-842, 1983). Back then, a multi-mode fiber with a large core outer diameter of about 50 .mu.m was mainly used, and such a plastic ferrule was applicable in connecting multi-mode fibers. The plastic ferrule, however, cannot be applied to a single-mode fiber with a core outer diameter of 8 to 10 .mu.m which is in current use for communication. Moreover, the ferrule, prepared by transfer molding, requires post-treatment such as flash trimming or heat-treatment after molding. From the viewpoint of economy during a manufacturing process, therefore, the plastic ferrule has not been fully satisfactory. An attempt was made to prepare a plastic ferrule by the use of a cost-effective injection molding technique. For this purpose, plastic materials such as PPS (polyphenylene sulfide) with relatively high thermal resistance and fluidity were considered.
For example, the following ferrules were investigated:
1. Biconical connector (ATT: W. C. Young et al., Proc. IWCS, 1981, p. 411-418)
Prepared by transfer molding of an epoxy resin. As in the study by Yoshizawa et al., the molding cycle was long, and required post-treatment such as deflashing. The manufacturing efficiency was too low to achieve cost reduction. For a single-mode fiber, the connecting properties were unsatisfactory.
2. A ferrule whose contour material was a hollow pipe and whose inside was fitted with a resin by transfer molding (Sumitomo Electric Industries, Ltd.: K. Kashihara et al., Proc. IWCS, 1987, p. 379-381)
Prepared by insert molding. Too laborious to prepare an economical ferrule at a low manufacturing cost.
Since the 1980's, the molding accuracy of injection molding has improved. Thus, a study was initiated using a more economical injection molding method (continuous production is easy and no post-treatment such as deflashing is required). Frequently studied molding materials were liquid crystal polymers (LCP) and polyphenylene sulfide (PPS).
3. A ferrule by injection molding using LCP (Du Pont: M. H. Johnson et al., SPIE Proc., vol. 1992, p.47-53, 1993)
Required the removal of a core pin from the ferrule after each molding shot, thus incapacitating continuous production. In addition, reliability in various environments was not entirely satisfactory.
4. A plastic ferrule configured to have a concave groove in the end face of the ferrule (NEC Corp.: K. Harada et al., Japanese Patent Application Laying-open No. 1-243005).
Has a concave groove in the ferrule end face, thus making the grinding of the ferrule end face difficult, and decreasing the strength of the fiber insertion hole portion.
The foregoing ferrules did not satisfy the dimensional accuracy, mechanical strength, or reliability for various parameters, capable of accommodating the connection of single-mode fibers. For example, the dimensional accuracy of a ferrule that is required for the connection of single-mode fibers has been shown to have to take the values listed in Table 1. Such a highly accurate ferrule has not been put into practical use.
The connecting properties of an optical connector can be evaluated by the connection loss and the return loss of the ferrule end face. The connection loss has to be not more than 0.5 dB, and the return loss must be 25 dB or more (Physical Contact (PC) grinding conditions) or 40 dB or more (Advanced Physical Contact (AdPC) grinding conditions).
Reliability tests have pointed out deterioration of the connecting properties that occurred in a high temperature or high humidity environment. The cause of this problem has been presumed to be that since a plastic ferrule has an optical fiber insertion hole length as shown in FIG. 2 of 1 to 2 mm (about 8 mm for a zirconia ferrule) according to the molding method, the adhesion between the optical fiber and the ferrule is insufficient under high temperature or high humidity conditions.